Common interferometer designs use a beam to measure characteristics of a surface that is being imaged.
U.S. Pat. No. 5,671,050 suggested using two different beams to measure information from the object. The two beams are reflected at different angles to the surface, and the distance to the object surface can be scanned.
In addition to the patent described above, U.S. Pat. Nos. 3,958,884, 5,502,562, and 5,011,280 teach interferometry techniques.
The present application teaches a dual beam interferometer with a number of additional advantages. One advantage is reference and signal beams may automatically be of the same magnitude independent of the reflectivity of the object. In one embodiment, two parallel beams are reflected from an object, and the spacing between the beams may be scanned. Imaging may be done with very small numerical aperatures which allows long working distance and smaller optics.